code: def test_return_na_data_method(df_na):
      # test with vars
      imputer = DropMissingData(
          threshold=0.5, variables=["City", "Studies", "Age", "Marks"]
      )
      imputer.fit_transform(df_na)
      X_nona = imputer.return_na_data(df_na)
      assert list(X_nona.index) == [2, 3]
  
      # test without vars & threshold
      imputer = DropMissingData()
      imputer.fit_transform(df_na)
      X_nona = imputer.return_na_data(df_na)
      assert list(X_nona.index) == [2, 3, 5]
  
file_path: feature_engine/tests/test_imputation/test_drop_missing_data.py
imports: import pytest
  from feature_engine.imputation import DropMissingData
label_desc: Use image node_id 4 for calling a global function with example usage: test_return_na_data_method(df_na) without return types